The present invention relates to a new and distinct variety of mandarin tree named ‘C4-15-19’. ‘C4-15-19’ is a triploid hybrid from an interploid cross of diploid monoembryonic ‘Sugar Belle’®, patented as ‘LB8-9’ (U.S. Plant Pat. No. 21,356) tangelo (female parent) crossed with an allotetraploid somatic hybrid of ‘Nova’ mandarin hybrid (unpatented, Clementine×Orlando)+‘Succari’ sweet orange (Citrus sinensis L. Osbeck) (male parent, unpatented), obtained via embryo rescue. Two original trees exist in Lake Alfred, Fla., one of which is grafted to ‘Carrizo’ (unpatented) citrus rootstock, and the other of which is grafted to a somatic hybrid of sour orange+‘Flying Dragon’ (unpatented). ‘C4-15-19’ was first asexually reproduced in Lake Alfred, Fla. Trueness-to-type through asexual propagation was demonstrated by topworking (grafting) onto the somatic hybrid rootstock sour orange+‘Carrizo’, also located in Lake Alfred, Fla.